If you, like me, think that evdev and xinput should somehow help you to disable the touchpad without the need of some daemon or gui tool, you're right!
Having a look at the output of xinput list is already looks promising:
[18:11] kr:~% xinput list
⎡ Virtual core pointer id=2 [master pointer (3)]
⎜ ↳ Virtual core XTEST pointer id=4 [slave pointer (2)]
⎜ ↳ Macintosh mouse button emulation id=7 [slave pointer (2)]
⎜ ↳ TPPS/2 IBM TrackPoint id=8 [slave pointer (2)]
⎜ ↳ SynPS/2 Synaptics TouchPad id=9 [slave pointer (2)]
⎣ Virtual core keyboard id=3 [master keyboard (2)]
↳ Virtual core XTEST keyboard id=5 [slave keyboard (3)]
↳ ThinkPad Extra Buttons id=6 [slave keyboard (3)]
↳ AT Translated Set 2 keyboard id=10 [slave keyboard (3)]
↳ Integrated Camera id=11 [slave keyboard (3)]
↳ Sleep Button id=12 [slave keyboard (3)]
↳ Video Bus id=13 [slave keyboard (3)]
↳ Power Button id=14 [slave keyboard (3)]
[18:11] kr:~%
Having a deeper look at device 9, the touchpad, with xinput list-props 9 reveals an interesting setting:
[18:12] kr:~% xinput list-props 9
Device 'SynPS/2 Synaptics TouchPad':
Device Enabled (125): 0
Device Accel Profile (244): 0
Device Accel Constant Deceleration (245): 1.000000
Device Accel Adaptive Deceleration (247): 1.000000
Device Accel Velocity Scaling (248): 10.000000
Evdev Reopen Attempts (242): 10
Evdev Axis Inversion (249): 0, 0
Evdev Axis Calibration (250): <no items>
Evdev Axes Swap (251): 0
Axis Labels (252): "Abs X" (262), "Abs Y" (263), "Abs Pressure" (264), "Abs Tool Width" (265)
Button Labels (253): "Button Left" (126), "Button Unknown" (243), "Button Right" (128), "Button Wheel Up" (129), "Button Wheel Down" (130)
Evdev Middle Button Emulation (254): 2
Evdev Middle Button Timeout (255): 50
Evdev Wheel Emulation (256): 0
Evdev Wheel Emulation Axes (257): 0, 0, 4, 5
Evdev Wheel Emulation Inertia (258): 10
Evdev Wheel Emulation Timeout (259): 200
Evdev Wheel Emulation Button (260): 4
Evdev Drag Lock Buttons (261): 0
[18:12] kr:~%
If you expect something like xinput set-prop even without reading the manpage, you are right:
[18:12] kr:~% xinput set-prop 9 125 0
This disables the touchpad and can be integrated into .xinitrc very well!